1. Field of the Invention
The present invention relates to a video signal processing circuit and, more specifically, to a video signal processing circuit for a video camera which separates a video signal obtained by AD conversion of an output of an image pickup device to a luminance signal and a color signal for output.
2. Description of the Background Art
Recently, video cameras called cam corders have been presented and various types of such video cameras in which video signal processing is carried out digitally have come to be available these days. Further, digital type video signal processing circuits having functions of auto focus for automatical focus adjustment, auto exposure for automatically adjusting exposure, auto white balance for automatical tone adjustment and of hand shake correction for correcting unnecessary motion of the picked-up image caused by hand shake have been proposed.
A conventional digital type video signal processing circuit is disclosed, for example, in Nikkei Electronics Feb. 3, 1992 (No. 546), pp. 237-241.
FIG. 3 is a block diagram showing a whole structure of a video signal processing circuit disclosed in the above mentioned article.
As shown in FIG. 3, the video signal processing circuit receives an analog video signal from a CCD type solid state image pickup device 1, separates the received signal to a luminance signal Y and color signals R-Y and B-Y and outputs the same.
The video signal processing circuit includes a correlated double sampling circuit (CDS) 2, a gamma correction circuit 3, an AD converter 4, Y/C signal processing circuit 5, a motion detecting circuit 6, a zoom processing circuit 7, field memories 8, 9, 10 and 11, a memory control circuit 12, a DA converter 13 and a filter 14.
Correlated double sampling circuit 2 removes noise component from the analog video signal received from solid state image pickup device 1. Y/C signal processing circuit 5 separates the digital video signal received from AD converter 4 into a luminance signal Y and a color signal C. Y/C signal processing circuit 5 further carries out signal processings such as aperture correction on luminance signal Y and white balancing on color signal C, in cooperation with a microcomputer 15. Motion detection circuit 6 compares video data including the luminance signal Y and the color signal C received from AD converter 4 with video data which is one field preceding, and detects direction and amount of motion of the object. Motion detection circuit 6 and field memories 8, 9 and 10 realize hand shake correction in cooperation with a microcomputer 16. DA converter 13 carries out DA conversion of video data which has been subjected to hand shake correction, and outputs an analog luminance signal Y and two color signals R-Y and B-Y.
In the video signal processing circuit, auto focus control is carried out by a gain control circuit 17, a microcomputer 18 and a focus motor driving circuit 19 based on an analog video signal received from gamma correction circuit 3.
Meanwhile, auto exposure control is carried out by a exposure motor driving circuit 20 based on the luminance signal Y received from Y/C signal processing circuit 5.
Generally, auto focus control in a video camera is carried out in the following manner. Namely, after an analog video signal is subjected to AD conversion, high frequency component thereof is extracted by high pass filter. The high frequency component is digitally accumulated at a prescribed period in each of a plurality of areas obtained by dividing the picked-up image plane. Based on the accumulated value, a prescribed operation processing is done by the microcomputer, and the result of the operation processing is applied as an auto focus control signal to a focus motor which drives the optical system. The reason why the high frequency component is extracted by the high pass filter is that the high frequency component included in the video signal is increased as the image is brought into focus.
Auto exposure control in a video camera is carried out in the following manner. Namely, after an analog video signal is AD converted, low frequency component is extracted by a low pass filter. Then, the low frequency component is accumulated digitally at a prescribed period in each of a plurality of areas obtained by dividing the picked-up image plane. Based on this accumulated value, a prescribed operation processing is done by a microcomputer, and the result of the operation processing is applied as an auto exposure control signal to an iris motor driving the diaphragm.
Further, auto white balance control in a video camera is carried out in the following manner. Namely, an analog video signal is AD converted and accumulated digitally at a prescribed period in each of a plurality of areas of a divided picked-up image plane. Based on the accumulated value, a prescribed operation processing is done by a microcomputer and the result of the operation processing is fed back as an auto white balance control signal to Y/C signal processing circuit and the like.
A recursive type noise reducer such as shown in FIG. 4 is known as a device for reducing noises included in the video signal. As shown in FIG. 4, the noise reducer includes two multipliers 21 and 22, one adder 23 and a field memory 24.
The noise reducer reduces noises appearing at random as time passes, and a coefficient K of multiplier 21 is controlled to be in the range of "0&lt;K&lt;1" corresponding to the motion of a motion picture. In case of a still image, the coefficient K of multiplier 21 is made close to "0" so as to provide larger effect of noise reduction.
In the above described video signal processing circuit, auto focus control is based on the video signal which has been subjected to gamma correction by the gamma correction circuit 3. Since gamma correction is non-linear processing, it makes precise auto focus control difficult. Especially in this video signal processing circuit, auto focus control is carried out based on the video signal before the luminance signal Y and the color signal C are separated, and therefore precise auto focus control is made more difficult by the unnecessary color signal C component.
Further, since auto exposure control is also carried out based on the luminance signal Y after gamma correction, precise auto exposure control is difficult.
Meanwhile, in the above described video signal processing circuit, most cf the blocks are formed by separate IC chips. It is generally proposed to incorporate a portion of the digital signal processing circuit including AD converter 4, Y/C signal processing circuit 5, zoom processing circuit 7, memory control circuit 12 and DA converter 13 on one IC chip.
However, in that case, it is necessary to separately provide an accumulating circuit and a high pass filter for auto focus control outside the IC chip. The same also applies to auto exposure control.
Further, in that case, accumulation must be done by using an analog video signal input to the IC chip or by using a digital luminance signal output from the IC chip.
In the former method using the analog video signal, an AD converter for AD conversion of the analog video signal must be separately provided. In addition, since the video signal does not have its luminance signal Y and color signal C separated therefrom, precise auto focus control or precise auto exposure control cannot be done because of the color signal component or the luminance signal component included in the video signal.
Meanwhile, in the latter method using the digital luminance signal, since an aperture signal is added to the digital luminance signal and non-linear operation has been effected thereon, it is difficult to carry out precise auto focus control or auto exposure control.
Further, in the video signal processing circuit described above, auto focus control and auto exposure control are carried out based on the signal before hand shake correction. Therefore, the accumulated value output from the accummulating circuit fluctuates because of the hand shake, which makes precise auto focus control and auto exposure control difficult.
It is preferred that the aforementioned recursive type noise reducer is provided in the video signal processing circuit. However, it newly requires an expensive field memory 24.